To enhance expression of foreign gene by the novel expression vector, pBmKSK1, of Bombyx mori nuclear polyhedrosis virus, E. coli $eta$-galactosidase gene expressing recombinant virus was infected in BmN-4 cells and various concentrations of silkworm hemolymph were added to the recombinant virus-infected BmN-4 cells containing fetal bovine serum. The expression efficiency of foreign gene was determined by $eta$-galactosidase activity in the culture media. The results showed that the silkworm hemolymph was effective to expression of foreign gene in the BmN-4 cells, suggesting that the silkworm hemolymph could be substituted for fetal bovine serum in the BmN-4 cells to enhance expression of foreign gene.